1. Field of the Invention
The present invention relates to a method for fabricating a thin film transistor (TFT) used as an active element of a liquid crystal display (LCD) device, and more particularly, to a method for manufacturing a TFT that in-advance prevents active patterns from forming in the unevenly crystallized portion of a polycrystalline silicon layer. Further, the present invention relates to a substrate and a TFT fabricated using such a method.
2. Description of the Related Art
Recently, demands for flat panel displays increase rapidly as the video equipment, such as a high definition television develops.
LCDs are one of the representative flat panel displays, and are gaining in popularity since they consume less power and provide a high access speed, which electroluminescene display (ELD), vacuum fluorescence display (VFD), or plasma display panel (PDP) has failed to provide.
There are two types of LCDs; an active type and a passive type. An active type LCD has a high speed, an excellent view angle and a high contrast, since each pixel therein is controlled by active elements, such as a thin film transistor (TFT). Such LCDs are more suitable for a high definition TV that requires a resolution with pixels of 1 million or more.
Recently, studies of TFT used as an active element of LCD have further developed as the significance of TFT grows. Specifically, studies are concentrated on the techniques that employ polycrystalline silicon for TFTs. This is because polycrystalline silicon has mobility characteristics approximately 100 times or more excellent than those of the commonly used amorphous silicon.
Since polycrystalline silicon has excellent mobility characteristics, TFTs using such a polycrystalline silicon may serve not only as a switching element, but as an embedded driving circuit such as an inverter.
A general architecture of TFT using such a polycrystalline silicon is disclosed in U.S. Pat. No. 5,780,326 entitled "Fully planarized thin film transistor and process to fabricate same", U.S. Pat. No. 5,705,424 entitled "Process of fabricating active matrix pixel electrode", U.S. Pat. No. 5,583,366 entitled "Active matrix panel", U.S. Pat. No. 5,499,124 entitled "Polysilicon transistors formed on an insulation layer which is adjacent to a liquid crystal material" and in U.S. Pat. No. 5,393,682 entitled "Method for making tapered poly profile for TFT device manufacturing".
To employ polycrystalline silicon for TFT, amorphous silicon is deposited onto a glass substrate. Subsequently, the deposited amorphous silicon layer is scanned by an excimer pulse laser beam using XeCl, KCl, ArF, etc. so that the amorphous silicon layer is crystallized to a polycrystalline silicon layer. In such a case, the amorphous silicon layer is heated by absorbing the laser beam, and the heated amorphous silicon layer is then rapidly crystallized, thereby forming a polycrystalline silicon layer with a stable structure.
As described above, when an amorphous silicon layer is crystallized to a polycrystalline silicon layer using a laser beam, only the surface of the glass substrate is heated by the rapid scanning of the laser beam. Therefore, a polycrystalline silicon layer having excellent mobility characteristics can be obtained without damaging the glass substrate.
Methods of crystallizing amorphous silicon using a laser are disclosed in U.S. Pat. No. 5,589,406 entitled "Method of making TFT display", U.S. Pat. No. 5,306,651 entitled "Process for preparing a polycrystalline semiconductor thin film transistor", U.S. Pat. No. 5,372,836 entitled "Method of forming polycrystalline silicon film in process of manufacturing LCD", U.S. Pat. No. 5,403,762 entitled "Method of fabricating a TFT", U.S. Pat. No. 5,403,772 entitled "Method for manufacturing semiconductor device", and in U.S. Pat. No. 5,472,889 entitled "Method of manufacturing large sized thin film transistor liquid crystal display panel.
On the polycrystalline silicon layer formed through the crystallization of amorphous silicon layer, a gate electrode, and a source/drain electrode are formed. The source/drain electrode is electrically connected to a pixel electrode of Indium Tin Oxide (ITO), thereby fabricating a TFT.
However, such a conventional method of fabricating a TFT has several problems.
As described above, in order to use polycrystalline silicon for a TFT, a process of crystallizing the already formed amorphous silicon layer to a polycrystalline silicon layer using an excimer pulse laser beam is pre-required.
However, here, a portion of the amorphous silicon layer, for example, the portion exposed to an edge of the laser beam, has an extremely uneven crystallization than the portion exposed to the center of the laser beam. This is because the laser beam employed for the crystallization of the amorphous silicon has a high energy density at its center, and a low energy density at its edge.
When the laser beam scanning completes to crystallize the amorphous silicon layer to the polycrystalline silicon layer, some areas of the completed polycrystalline silicon layer, for example, a pixel area, a source drive area, and a gate drive area, have active patterns having a predetermined width.
As aforementioned, since laser beams have an unbalanced energy density between its center and its edge, the amorphous silicon layer crystallized by the laser beam also has a portion exposed to the edge of the laser beam, which has an extremely unstable crystallization than those exposed to the center. Accordingly, the amorphous silicon layer exposed to the edge of the laser beam cannot be completely crystallized to a polycrystalline silicon layer, and thus has an unstable state.
Here, if a portion of active patterns (to be formed later) is formed in the unevenly crystallized area, such active patterns may not work well.
To address this problem, methods for preventing active patterns from being formed in the unevenly crystallized area of the polycrystalline silicon layer are required. However, the conventional method failed to suggest such a method.
When active patterns are formed in the unevenly crystallized area of the polycrystalline silicon layer, TFTs formed on those active patterns will not work well.